U.S. Department of Commerce, National Oceanic & Atmospheric Administration
(NOAA), National Geophysical Data Center
& World Data Center A for Marine Geology & Geophysics
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EXPLORING THE OCEAN BASINS WITH SATELLITE ALTIMETER DATA
D.T. Sandwell, Scripps Institution of Oceanography & W.H.F. Smith, NOAA
Geosciences Laboratory
FIGURES AND CAPTIONS FROM THE REPORT
(maps produced using GMT software)
Figure 1: Global gravity anomaly map derived from 4.5 years of Geosat altimeter
data and 2 years of ERS-1 altimeter data. Green areas have essentially normal
gravity, yellow-orange-red hues represent increasingly stronger-than-normal
gravity and blue-violet-magenta colors represent increasingly
weaker-than-normal gravity. The gray image is an "artificial illumination" of
the gravity field, illuminated from the north (top). The full resolution
gravity data are available by anonymous ftp from baltica.ucsd.edu. A large
color poster is available from Scripps Geological Data Center (see
http://julius.ngdc.noaa.gov/mgg/announcements/announce_predict.html for details
on how to order the map or obtain more information about it).
Figures 2, 3, 4
The Fracture Zones:
The heavy curved line running from north to south mid-way between Africa and
the Americas is the location of the mid-Atlantic ridge. The Earth's tectonic
plates move apart along this line and lava pours out of the opening, creating
new sea floor. The distance between a point in North America and a point in
Africa grows larger by about one inch per year, or a little slower than human
fingernails grow. The path of the ridge jogs back and forth at offsets called
transform faults. These transform faults leave scars on the ocean floor called
fracture zones, and the motions of the plates can be traced by following these
fracture zones.
Atlantic history:
Early in the Mesozoic Era (the age of dinosaurs) the continents were all
assembled in one land mass called Pangaea. Little by little it broke up. North
America broke away from Africa beginning about 180 million years ago in the
early Jurassic period. Later, around 110 million years ago in the early
Cretaceous, South America broke away from Africa. The two americas took
slightly different paths, as these gravity images show. The fracture zones
between the Oceanographer Fracture Zone and the Fifteen Twenty Fracture Zone
all appear to follow the same path, showing the movement of North America away
from Africa. The fracture zones south of the Four North Fracture Zone,
including St. Paul, Romanche, and others south of the bottom of this map,
follow a different path, the path of South America away from Africa.
What is new here:
The motion that must have taken place between North America and South America
as both of them left Africa has not been easy to see until now. Now that we can
see all the fracture zones clearly, it is apparent that there is an area where
these are parallel to the northern group in some places and the southern group
in other places. In particular, the area of the large numeral "1" in the key
shows several fracture zones coming together. On the other side of these
fracture zones at "2" there are some large gravity anomalies which look like
deformation structures, as if the sea floor may have been pinched between North
and South America. Some geologists have also hypothesized about motion between
Africa and Europe, and also whether the Iberian peninsula has moved
independently of Africa and Europe. By studying this new data in the area
between "3" and Gibraltar it should be possible to test this hypothesis.
Some geologists believe that the zigs and zags of a mid-ocean ridge are
inherited from the geometry of the initial breakup of the continents. Others
believe that the mid-ocean ridge is a dynamic system which evolves to adjust to
changes in plate motion. If the ridge geometry remains fixed then the distances
between fracture zones should remain constant across the ocean basin. Careful
study of these gravity images should settle the question. It appears that these
data lend support to the dynamic idea. The fracture zones are not all parallel,
and V-shaped structures (one large one just north of the Kane Fracture Zone)
can be seen along the ridge. These observations suggest that ridge axis segment
lengths grow and decay with time.
Figure 5: Schematic diagram of satellite altimeter measurement. h* is the
height of the satellite above the reference elipsoidal shape of the earth
determined by precisely tracking the satellite. h is the altitude of the
satellite measured with a radar. Variations in sea surface height reflect
massive features in the seafloor.
Figure 6-7: (bottom) Gravity anomaly over the Indian Ocean triple junction
derived from widely-spaced tracks of Seasat altimeter data. This represents the
best resolution that could be achieved by Seasat. The major limitation is wide
profile spacing due to the short 3- month lifetime of the Seasat satellite.
(top) Gravity anomaly over the Indian Ocean triple junction derived from Geosat
and ERS-1 altimeter data. The combination provides very dense track coverage
and thus improved resolution. A triple junction is a point where 3 tectonic
plates meet. This area is the intersection of 3 spreading ridges, the Central
Indian Ridge (toward north), the Southeast Indian Ridge (toward southeast) and
the Southwest Indian Ridge (toward southwest).
Figure 8: (Left) Color shaded relief image of sea surface gravity anomalies
over the Southwest Indian ridge computed from Geosat/GM and other altimeter
data (Right) Color shaded relief image of sea floor topography predicted from
the data at left. Both images are illuminated from the southeast.
Figure 9-11: Predicted seafloor depth over uncharted Foundation Seamounts.
Black lines show tracklines of research vessels where profiles of seafloor
depth were collected. This region was recently explored by a team of German and
French scientists who used these depth predictions to guide their survey.
Figure 12: Gravity field of Northeast Pacific Ocean This illustrates all of the
major geologic elements of the seafloor. The long fracture zones were produces
30-100 million years ago at a major seafloor spreading ridge that does not
exist today; it was subducted beneath North America. The Aleutian Trench (top)
reflects current subduction of the Pacific plate. Small and large (e.g.
Hawaiian Chain) seamount chains overprint the original fracture zone and
abyssal hill seafloor fabric.
Figure 13: Map of bathymetric survey lines collected by ships. This map shows
all data are in NOAA's National Geophysical Data Center, which is the World
Data Center A for this type of data, as of October 1995. The typical hole
between surveys in the southern oceans is about the size of the State of
Oklahoma. Furthermore, much of the data is old and of poor quality. Using
state-of-the-art equipment available today, it would take 125 years to map the
oceans with ships.
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